It is a well known form of exercise to create a resistance to muscular contraction or elongation. Exercise producing resistance may be provided by free weights, i.e., barbells or plates attached to a bar, or machines utilizing, for example, weight stacks, compressed air, hydraulics, magnets, friction, springs, bending flexible rods, rotating fan blades, mechanical dampers or the users own body weight. A conventional exercise with free weights, for example, involves a "positive" movement in which the muscle under training is contracting to lift a weight and a "negative" movement in which that muscle is elongating to lower the weight. Many exercise machines emulate the exercise movements used in free weight training.
There are many disadvantages to exercising with both free weights and these conventional exercise machines. For instance, free weights are potentially hazardous without a partner to "spot" the user, and it is difficult and time consuming to adjust the amount of weight to be used in order to perform a different exercise or to accommodate another person of differing strength. Various exercise machines tend to be heavy and/or bulky and do not offer the intensity, range-of-movement and variety of movement of free weights. Also, both free weights and weight machines cannot be used in a gravity-free environment, such as encountered by astronauts.
An alternative form of exercise utilizes inertia to provide exercise-producing resistance. Such exercise is based on the principle that force is required to rotationally accelerate a mass, i.e., to increase or decrease the rotational velocity of a mass. An inertial exercise device has several advantages over both free weights and conventional exercise machines. Less bulk is required because the difficulty of the exercise depends not only on mass but also on the angular acceleration of mass. No partner is required as with free weights. Further, an inertial exercise device does not require gravity.
Existing exercise devices utilizing inertia, however, suffer from several disadvantages. Many such devices provide only a positive work exercise. Further, it is often difficult to vary the resistance of inertial exercises. Finally, unlike free weights or some exercise machines, existing inertia-based exercise devices have difficulty providing a constant resistance and/or constant speed of movement.